This invention relates to new and useful improvements in rotary well drilling operations and provides the means for transmitting desired down-hole information to the surface by causing the drilling rig pump pressure to depart from its normal level in sympathy with digital signals derived at a down-hole location. This is accomplished by the venting of drilling fluid from the interior of the drill stem into the borehole annulus in a binary coded decimal format, by means of a valve that is caused to operate by the digital output of one or more down-hole transducers.
In exploring for crude oil and natural gas, it is the present practice to drill wells into the earth using a "rotary" drilling technique. Under these circumstances, the drilling apparatus includes means whereby a drill string (consisting of a number of sections of hollow pipe and having a drill bit connected to the lower end) is caused to rotate, while the amount of axial force applied to the bit is carefully controlled.
While the well is being drilled, it is common practice to circulate drilling fluid (some mixtures of which are colloquially known as "mud") down through the hollow drill string, through restrictive nozzles in the bit, and back to the earth's surface through the annulus of the borehole. Upon reaching the earth's surface, the drilling fluid (containing the cuttings from the drill bit) is allowed to flow through a screening device into a series of tanks from which it is recirculated through the borehole. Besides providing the vehicle whereby cuttings are returned to the surface, the drilling fluid acts to cool and lubricate the drill bit and the exterior of the drill string. Also, the drilling fluid provides a back pressure in the hole to more or less contain natural gas that may be encountered during the drilling process.
For the fluid to provide the required cleaning and cooling action with respect to the drill bit, the bit is constructed to include a number of nozzles (typically three) through which the fluid is forced at relatively high pressures. To accommodate the need for such drilling fluid pressures, the drilling apparatus includes pumps, piping, and the required swivel joint to permit the entry of the fluid into the rotating drill string.
It has become increasingly apparent to the producers of crude oil and natural gas, that means must be found to increase the efficiency of drilling operations in order to offset sharply rising costs. In part, the increased costs are due to the need to explore in geographical areas that are more and more remote from the markets for petroleum products.
Those skilled in the art of drilling wells recognize that the efficiency would improve significantly if some means could be found whereby they are advised of conditions at the bottom of the hole while drilling is in progress. For example, the bit may encounter a sloped sub-surface formation causing it to depart from the vertical and commence drilling at an angle. Or if excessive weight is inadvertently applied to the bit, the drill string will bend, again causing the hole to depart from the vertical. If the bore-hole is allowed to progress at an angle with respect to the vertical, the "target" zone may be missed altogether, and the drill string will be exposed to excessive wear due to the tangential rubbing action at the point of hole curvature.
To determine whether or not the bore-hole has departed from the vertical, it is the present practice to periodically stop drilling while a "survey" is conducted. Typically, a survey is carried out by lowering a recording inclinometer (EG: one in which the position of pendulum is photographed) down the interior of the drill string on electrically conducting wires.
Alternatively, an instrument responsive to inclination and triggered by a clockwork mechanism may be dropped into the drill string and recovered later. The former technique is expensive and time consuming, and necessitates the stopping of fluid circulation -- which emphasizes the possibility of the drill string becoming stuck in the hole. The latter technique will only provide information considerably after the fact, and if the instrument happened to encounter an obstruction en route to the bottom of the hole, the clockwork mechanism may have triggered the recording too soon, thereby providing seriously misleading information.
To facilitate further research into well drilling operations, it would be useful to have other information (EG: temperature, pressure, weight on bit, etc.) telemetered from the bottom of the hole while wells are being drilled. This information would be used to more accurately predict the performance of various types of bits, the likelihood of encountering a gas bearing formation, the optimization of table RPM and weight on the bit to achieve maximum penetration rate, the mixture of mud that should be used, and the like.
Clearly, then, an instrument that will effect the transmission of down-hole information to the earth's surface, dependably, without the use of wire lines, would provide drillers with the means to significantly improve the efficiency of their operations.
Numerous attempts have been made to build a device that will provide wireless transmission from a down-hole location to the earth's surface. For example, various systems based upon the transmission of acoustical waves through the steel drill string or through the drilling fluid have been described in the prior art. However, such factors as the attenuation of the signals by the drill string immersed in the drilling fluid, bubbles in the drilling fluid, the ambient noise in the drilling apparatus, the hostility of the down-hole environment (with respect to temperature, pressure, and vibration), the transfer of energy from transducers into the transmission medium, the need for relatively large amounts of electrical power in the down-hole apparatus, and the need for "rugged simplicity," have, insofar as the applicants are aware, precluded the reduction of such prior art to actual use.